University of Kentucky College of Agriculture, ASC-223 Food and Environment Cooperative Extension Service Inbreeding in Sheep Debra K. Aaron, Animal and Food Sciences

The inbreeding process itself is not responsible for undesirable genes; it merely permits the recessive genes to be expressed as a result of the increased homozygosity.

vigor, which is the advantage gained from crossing lines or breeds. Inbreeding also increases the frequency of abnormalities. For example, the spread of spider lamb syndrome in black-faced sheep is believed to be the consequence of inbreeding. Measurement of Inbreeding Inbreeding in an individual is mea- sured with the inbreeding coefficient. The inbreeding coefficient measures the per- nbreeding is broadly defined as the homozygous gene pairs and a decreased cent increase in homozygous gene pairs in Imating of individuals that are related. number of heterozygous (Aa) gene pairs. an individual relative to the average of the Strictly speaking, however, all animals This increase in homozygosity is breed. If an individual animal has an in- within a breed are related. So, in a sense, desirable, and inbreeding is good, if the breeding coefficient of 0.25, it is expected every purebred sheep producer practices gene the individual receives from each to have 25 percent more homozygous some degree of inbreeding. In most cases parent leads to superior performance. gene pairs than a non-inbred individual this relationship is very slight. Therefore, Unfortunately, however, most individu- from the same breed. As explained ear- inbreeding is more practically defined als carry undesirable recessive genes that lier, this increase in homozygosity is what as the mating of individuals more closely remain hidden unless he or she has a pair leads to inbreeding depression if the related than the average of the breed. This of them. Inbreeding brings to light these recessive alleles are detrimental. practice includes mating brother to sister, undesirable recessive genes. An inbred Theoretically, the inbreeding coef- sire to daughter and son to dam. individual is more likely to have gene ficient can have any value between 0 and pairs with identical members (in other 1.0, although it is unlikely for it to have a Effects of Inbreeding words, to be homozygous for any gene) value much above 0.5 in most flocks of on a Flock and, as a result, is more likely to express sheep or herds of cattle, goats or pigs. On undesirable genes. If this expression oc- the other hand, it is fairly easy to have it Inbreeding can have dramatic effects curs, inbreeding is bad because it results approach 1.0 in some plant species, where on a flock. These effects are the result of in the expression of undesirable traits. self-fertilization is possible. Also, there an increase in the pairing of identical On the average, animal performance are some lines of laboratory animals genes. When an individual receives an for traits such as fertility, prolificacy, with very high levels of inbreeding. It identical gene ( or , for example) from A a disease resistance, vigor and surviv- is unlikely that farm animals will have each parent, it is said to be homozygous ability declines with inbreeding. This extremely high inbreeding coefficients (AA or aa) for that pair of genes. If the decline is called inbreeding depression. because it takes several generations of parents are related, it is more likely they This phenomenon is well-documented full-sib or parent-offspring matings to have genes that are identical. So, inbreed- in all major livestock species. It is essen- have an inbreeding coefficient greater ing results in an increased number of tially the opposite of heterosis, or hybrid than 0.5.

Cooperative Extension Service | Agriculture and Natural Resources | Family and Consumer Sciences | 4-H Youth Development | Community and Economic Development Bracket A Arrow B Bracket A Arrow Pedigree Pedigree Pedigree Pedigree S S A S S X A X B B X X A D A D B C D D

C B

Figure 1. Half-sib mating scheme Figure 2. Full-sib mating scheme

The inbreeding coefficient is ex- top [sire] and bottom [dam] sides of the Then, using the equation presented pressed as follows: bracket pedigree and is indicated by a earlier, the inbreeding coefficient is cal- circle in the arrow pedigree). culated as: n+1 FX = φ [(½) (1 + FA)] When calculating the inbreeding n+1 n+1 X FX = (½) (1 + FA) + (½) (1 + FB) Where coefficient of individual , it is necessary = (½)3(1 + 0) + (½)3(1 + 0) to consider all possible paths by which F = Inbreeding coefficient of individual X = 0.25 X the sire and dam of individual X are con- Σ = Summation sign (means values in the nected. In the pedigree illustrated in brackets are to be added together) Again, because no pedigree informa- Figure 1, there is only one path by which tion is available on the two common n = Number of segregations (arrows) the sire (S) and dam (D) are connected: ancestors (A and B), their inbreeding coef- between the sire and dam in each F F separate path through the common S A D ficients, A and B, are assumed to be zero. ancestor The inbreeding coefficient reveals a much higher level of inbreeding when X results FA = Inbreeding coefficient of the com- Using the equation presented earlier, mon ancestor in each separate path. the inbreeding coefficientF ( X) is calcu- from the mating of full-sibs (Figure 2), as X A common ancestor is one that lated as: compared to the situation where results appears on both the sire and dam from the mating of half-sibs (Figure 1). side of the pedigree. Whenever a n+1 FX = (½) (1 + FA) common ancestor is inbred, his or = (½)3(1 + 0) Parent-Offspring Mating her inbreeding coefficient will have = 0.125 to be calculated before the inbreed- Another form of inbreeding involves ing coefficient of individual X can be parent-offspring mating. One type, for determined. Because no pedigree information is available on A, the common ancestor, FA example sire-daughter, is illustrated with is assumed to be zero. This inbreeding the pedigree in Figure 3. This example Inbreeding Systems F coefficient tells us that individual X is also includes an inbred ( S = 0.25) parent, S Some of the more common forms expected to have 12.5 percent more ho- because the sire ( ) resulted from a mat- of inbreeding can be used to illustrate mozygous gene pairs than a non-inbred ing between full-sibs. calculation and interpretation of the individual from the same breed. In the pedigree illustrated in Figure 3, inbreeding coefficient. there is only one path by which the sire Mating between Full Sibs (S) and dam (D) are connected: Mating between Half-Sibs This system can be depicted as shown S D The first inbreeding system involves in Figure 2. In this case S and D, the sire mating between half-sibs. This mating and dam of X, are full-sibs (that is, full Using the equation presented earlier, scheme is presented in Figure 1 in both brother and sister) because they have the the inbreeding coefficient is calculated as: bracket (traditional way to depict a pedi- A B same sire ( ) and dam ( ). n+1 FX = (½) (1 + FA) gree) and arrow forms. Here is how the inbreeding coefficient 2 S = (½) (1 + 0.25) In this pedigree (Figure 1), is the is determined. First, the two paths con- = 0.3125 sire of X and D is the dam of X, and these necting the sire and dam of X are drawn: two individuals are half-sibs (that is, half F S A D If S had been non-inbred ( S = 0), then brother and sister because they have FX would be 0.25. Consequently, the A one parent in common). Also, is the S B D inbreeding of S has contributed to the common ancestor (appears in both the inbreeding of X.

2 Uses of Inbreeding relationship between a parent and its linebreeding program should be an indi- offspring. However, because very close vidual that was superior in performance Inbreeding can sometimes benefit a matings were avoided—no mates shown and type but is no longer in service. If purebred sheep producer. Inbreeding in the arrow pedigree are more closely such an individual cannot be identified tends to subdivide a breed into families, related than half sibs—individual X is clearly, the purebred sheep producer which can be identified and crossed with only mildly inbred (FX = 12.5%). The line- should continue to avoid inbreeding. some small benefit. It also promotes an prepotency breeding depicted in Figure 4 illustrates increase in , which refers to a very deliberate strategy. The goal was Summary the ability of an individual, usually a to concentrate the genetic material of Although inbreeding does not cre- sire, to transmit the same inheritance to 5 X individual in individual while keeping ate undesirable recessive genes, it does different offspring, or, on other words, to the level of inbreeding low. uniformly “stamp” his characteristics on bring them to light. This approach can Linebreeding has the advantage of result in the expression of undesirable his progeny. This prepotency is a result maintaining genes from outstanding of the increase in homozygosity, as was traits, genetic deformities and, ultimately, individuals, usually sires, that are no inbreeding depression. Therefore, the described earlier. Because an inbred indi- longer available for breeding purposes. It vidual will have more homozygous gene disadvantages of inbreeding outweigh also enables a certain amount of family the advantages for all but a small number pairs than a non-inbred individual, there name recognition, which can be useful are fewer possible combinations for the of top purebred sheep breeders. Inbreed- when advertising breeding stock. At the ing limits flock improvement by reducing gametes (sperm in sires, eggs in dams). same time, at least two dangers may be The genetic makeup of gametes from the reproductive performance, vigor and associated with linebreeding: it is dif- survivability, and it restricts selection op- inbred individual will be less variable so ficult to follow a linebreeding program the offspring will be more similar. portunities by placing a disproportionate for any period of time without increasing emphasis on pedigree information. One of the most common uses of in- the amount of inbreeding in a flock, and breeding is linebreeding, which is a mating Generally, the recommendation is that there is the possibility of linebreeding to inbreeding should be avoided as much as system designed to maintain a high degree rams that are not genetically superior. of relationship to a genetically superior possible by any purebred producer who A linebreeding program should only does not have a clear understanding of ancestor without causing high levels of be started by a sheep breeder with a su- inbreeding. Linebreeding is a mild form of its use. Linebreeding can be an effective perior purebred flock and a fundamental tool for perpetuating the genes of an inbreeding. An example of linebreeding understanding of the benefits and dan- is represented with an arrow pedigree in outstanding ancestor, but it should only X gers of such a program. It should not be be used in superior flocks and only with Figure 4, where individual is linebred considered unless the flock is so high in to individual 5. Individual 5 shows up truly outstanding rams. Finally, mating X quality and performance that it is difficult of close relatives, such as parents with so often in the pedigree of individual to find outbred animals that will im- that the relationship between the two is offspring or brothers with sisters, should prove the genetic merit of the flock. The be avoided in all flocks. 47 percent, practically the same as the animal, usually a sire, at the center of the

1 1 3 S S 2 X 5 X 3 2 4 D D 4

Figure 3. Sire-daughter mating with an inbred parent Figure 4. A form of linebreeding where individual X is linebred to individual 5

Educational programs of Kentucky Cooperative Extension serve all people regardless of race, color, age, sex, religion, disability, or national origin. Issued in furtherance of Co- operative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Nancy M. Cox, Director, Land Grant Programs, University of Kentucky College of Agriculture, Food and Environment, Lexington, and Kentucky State University, Frankfort. Copyright © 2014 for materials developed by University of Kentucky Cooperative Extension. This publication may be reproduced in portions or its entirety for educational or nonprofit purposes only. Permitted users shall give credit to the author(s) and include this copyright notice. Publications are also available on the World Wide Web at www.ca.uky.edu. Issued 12-2014